Among the medical imaging modalities, ultrasound imaging technology is noninvasive, safe, affordable and generally easy to use. Ultrasound operates using a short burst mechanical wave transmitted into a patient's body. The echo reflected from structures in the patient's body is processed to form two dimensional or three-dimensional images for tissue and flow information. The frequency range for a noninvasive medical ultrasound imaging system is normally between 1 MHz to 15 MHz. The ultrasound images are normally displayed in real-time. Therefore, the operator or clinician can make an immediate diagnostic decision, or archive it for future comparison purposes.
As part of a given ultrasound data collection session, the data can be used to detect blood flow. Such flow detection can be performed using ultrasound Doppler imaging. A probe that includes one or more transducers can be used to transmit acoustic waves to and receive scattered waves from a sample of interest. The waves or echoes backscattered from the sample include information about the sample and its properties and components. When it comes to detecting blood flow in a sample, the backscattered acoustic waves can be compared to a frequency reference to determine if a Doppler frequency shift occurred. Such a frequency shift can occur in the backscattered waves as a result of moving backscattering elements from the sample such as blood cells within an artery.
Further, such a frequency shift, which can also correspond to a phase shift, when detected using the ultrasound system, can be correlated with the velocity of the blood flow in the sample. The blood velocity is calculated by measuring the phase shift. The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer.
Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over a gray scale anatomical B-mode image. Typically, a color flow mode displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image to represent each sample volume's velocity. When arteries are adjacent, regions of overlap in an image can be challenging to resolve and result in ambiguities relating to flow direction. A need therefore exists for methods to resolve such ambiguities and improve upon color flow mapping techniques. The embodiments of the invention address these needs and others.